As is known in the art, there are many of types of artificial light sources such as incandescent, fluorescent, and high-intensity discharge (HID) light sources. Fluorescent and HID light sources or lamps are generally driven with a ballast which includes various inductive, capacitive and resistive elements. The ballast circuit provides a predetermined level of current to the lamp which causes the lamp to emit light. To initiate current flow through the lamp, the ballast circuit may provide relatively high voltage levels, e.g., a strike voltage, that differ from operational levels.
One type of ballast circuit is a magnetic or inductive ballast. One problem associated with magnetic ballasts is the relatively low operational frequency which results in a relatively inefficient lighting system. Magnetic ballasts also incur substantial heat losses thereby further reducing the lighting efficiency. Another drawback associated with magnetic ballasts is the relatively large size of the inductive elements.
To overcome the low efficiency associated with magnetic ballasts, various attempts have been made to replace magnetic ballasts with electronic ballasts. One type of electronic ballast includes inductive and capacitive elements coupled to a lamp. The ballast provides voltage and current signals having a frequency corresponding to a resonant frequency of the ballast-lamp circuit. As known to one of ordinary skill in the art, the various resistive, inductive and capacitive circuit elements determine the resonant frequency of the circuit. Such circuits generally have a half bridge or full bridge configuration that includes switching elements for controlling operation of the circuit.
Conventional ballasts generally provide particular voltage and current levels adapted for a single lamp size. Thus, a ballast is only useful for one particular lamp. As known to one skilled in the art, the diameter of the lamp determines the level of current that flows through the lamp. That is, lamps of eight feet, four feet, two feet and one foot all pass about the same amount of current, provided that the lamps have the same diameter. The voltage drop across the lamp, however, varies in accordance with the length of the lamp. The longer the lamp, the greater the voltage drop across the lamp. It would be desirable to provide a ballast that can energize any lamp in a family of lamps where each lamp has the same diameter and a different length.
Another drawback to some known ballast circuits is associated with initiating, or attempting to initiate, current flow through the lamps. One type of ballast initially operates in a so-called rapid start mode to establish current flow through the lamp and thereby cause the lamp to emit light. In rapid start mode, the ballast heats the lamp filaments with a predetermined current flow through the filaments prior to providing a strike voltage to the lamp. Thereafter, the ballast provides operational levels of voltage and current to the lamp as it emits visible light. However, in the case there a lamp does not light, such as a lamp that is only marginally operational, excessive energy levels can be generated by the circuit. High voltages and currents can stress the circuit components and thereby reduce the useful life of the ballast. It would, therefore, be desirable to provide a ballast that detects and eliminates excessive signal levels that can occur when a lamp fails to start. It would also be desirable to provide a ballast circuit that, when attempting to light the lamp, applies a strike voltage to the lamp at predetermined intervals to reduce stress on the ballast circuit components.